As a friction stir welding method, JPS62183979A discloses a technique of welding metal materials by rotating both of or one of a pair of metal materials to generate frictional heat in the metal materials to soften the material, and stirring the softened portion to cause a plastic flow.
However, with this technique, since the metal materials which are the subjects to be welded are rotated, the shape and size of the metal materials to be welded are limited.
On the other hand, JPH7505090A proposes a method of continuously welding working materials in a longitudinal direction using the heat and plastic flow generated between the tool and working materials by inserting a tool made of material substantially harder than the working materials into an unwelded portion of working materials and moving the tool while rotating it.
The friction stir welding method described in JP '979 is a method of welding working materials together by rotating the working materials and using frictional heat generated between the working materials. On the other hand, with the friction stir welding method disclosed in JP '090, steel sheets or plates can be welded together by moving the tool while rotating it in a state where the welding members are fixed. Therefore, that technique is advantageous in that continuous solid state bonding can be performed in the longitudinal direction of the members even on members which are substantially infinitely longer in the welding direction. Further, since solid state bonding is performed by utilizing the metal plastic flow caused by the frictional heat generated between the rotational tool and the welding materials, steel sheets or plates can be welded together without melting the portion to be welded. In addition, the technique of JP '090 has many advantages. For example, there is less deformation after welding because of the low heating temperature, there are fewer defects because the welded portion is not melted, and a filler material is not required.
Use of the friction stir welding method is spreading in the fields of aircraft, ships, railway cars, automobiles and the like, as a method of welding low melting point metal materials including aluminum alloy or magnesium alloy. This is because, with these low melting point metal materials, it is difficult to obtain satisfying characteristics in the welded portion by the conventional arc welding method, but it is possible to enhance productivity and obtain welded portions of high quality by applying the friction stir welding method.
On the other hand, by applying a friction stir welding method to structural steels mainly applied as materials for structures such as buildings, ships, heavy machinery, pipelines, automobiles and the like, it is possible to avoid solidification cracking and hydrogen cracking which have been a problem in conventional melt-welding methods and, since the microstructural change of the steel material will be suppressed, excellent joint characteristics are expected. Further, it is also expected that, since purified surfaces are created by stirring the welding interface with a rotational tool and the purified surfaces are contacted to one another, a preparatory step such as diffusion bonding is not required. As described above, many advantages are expected by applying the friction stir welding method to structural steels. However, because of problems regarding welding workability which remain to be solved such as suppression of defect generation at the time of welding or the increase of the welding rate, the friction stir welding method is not as widely used compared to low melting point metal materials.
As described in JP2003532542A and JP2003532543A, high abrasion resistance materials such as polycrystalline cubic boron nitride (PCBN) or silicon nitride (SiN4) are currently used as the rotational tool in friction stir welding of structural steel. However, those ceramics are brittle and, therefore, sheet thickness and processing conditions of the steel sheets or plates to be welded are severely restricted to prevent damages to the rotational tool.
Further, JP200394175A and JP2005288474A disclose, for the purpose of improving welding workability, a welding method including a heating unit other than the frictional heat generated between the rotational tool and the welding materials.
For example, JP '175 discloses a heating device for the friction stir welding method provided with a heating unit using an induction heating device where an increase of the welding rate and the elimination of cracks in the welded portion are sought by heating the working materials before and after the welding thereof.
Further, JP '474 discloses a friction stir welding device provided with a heating unit using a laser device where an increase in the welding rate is sought while suppressing microstructural change around the heating region caused by pre-heating, by partially heating the working materials right before welding is performed.
However, with the techniques of JP '175 and JP '474, the surface temperature, depth or the like of the heating region of the working material heated by the heating before the welding have not been taken into account and, therefore, sufficient welding workability cannot be obtained. Further, there were cases where excessive heating caused a change in the microstructure around the heating region and provided an adverse effect on welding workability, particularly on joint strength. Therefore, in the present circumstances, a practical friction stir welding method to obtain a sufficient strength and improving welding workability has not been discovered.
It could therefore be helpful to advantageously resolve the plastic flow failure caused by insufficient heating of working materials to obtain a sufficient strength and improve welding workability when performing friction stir welding on structural steel.